As peace doves fluttered across the ghostly skeleton of Hiroshima's Atomic Bomb Dome last Friday and a sombre Buddhist temple bell tolled, the presence of one man added to the poignancy of what is always a bitter and beautiful ceremony. That man was John Roos, US ambassador to Japan and the first representative of the US government to attend the memorial in the 65 years since the atomic bomb was dropped.

In one sense, President Barack Obama's decision to send the ambassador was straightforward. Despite the risk of creating a backlash in the US, where some critics thought Mr Roos's presence could be misinterpreted as an apology, Hiroshima is a potent symbol for the nuclear disarmament that the US president has championed. Ever since Mr Obama's Prague speech calling for a nuclear-free world last year, officials from Hiroshima and Nagasaki have been pressing him to visit their cities, an invitation he has not yet accepted.

There may also have been a more emotional motive still. Certainly, one can make a case, though not a definitive one, that dropping the two bombs shortened the war and actually saved lives. Yet, it is hard to read accounts of the bombings and their aftermath – in which civilians were burnt alive or vaporised, or died over months as maggots bred in their festering wounds – without some sense that the use of such terrible weapons is wrong.